The present invention relates to a low input impedance voltage protection system for electronic measurement devices and particularly to such a system that will switch to a high impedance state when the input voltage exceeds a desired range.
A problem in voltmeter design is protecting circuitry from input-overvoltage while keeping the sensitivity and noise suppression at acceptable levels to allow precise voltage measurements of DC and AC signals. Due to power surges, noise or misapplication of measurement probes, a voltmeter may be measuring values in the millivolt range and moments later be presented with a 1000 volt signal at its input terminals. Circuitry capable of measuring small voltage levels typically is not able to survive the sudden appearance of large voltage inputs without damage to the circuit components. A known method of providing input voltage protection employs a large resistance in series with the input signal to limit peak current and clamping diodes between the input signal and positive and negative voltage rails to limit peak input voltage. While that type of circuit protects the measurement device from excessive voltage and current swings, it creates sensitivity and noise problems. Large input resistors facilitate noise pick-up and generation and also create errors measuring high frequency AC signals. For example, a one megohm resistor when coupled to a stray capacitance of about ten picofarads will limit precise measurements to about one kilohertz. Also a one megohm input resistor will produce 130 nanovolts/root hertz thermal noise. Large DC errors are created when bias current from meter amplifier inputs or leakage current from over-voltage protection diodes flow through large input resistors. It would be desirable to provide a low impedance input for signals within a specified voltage range with switching to a higher impedance state when large voltage levels appear at the input.